Modulacin Analgica AMFM Parte II

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Modulación Analógica (AM-FM) (Parte II)

• Cx Eléctricas 08 E.Tapia

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FM Discriminator S2N
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Cont

• The carrier power has noise quoting effect in FM
• Recall that
• The average signal transmitted power is kf2P

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How can we improve S2N in FM?
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The conclusion

• FM provides a mechanism for the exchange of
  improved noise performance by increased
  transmission bandwidth
• FM can also reject other FM signals closed to the
  carrier frequency provided interferent signal are
  weaker w.r.t. the target FM input

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Threshold Effect in FM

• Assumption
• Carrier to Noise ratio at the discriminator input
  gtgt 1
• Violation to this assumption
• FM receiver breaks. From breaks to sputtering
  sounds. The formula does not hold.

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No signal but Noise

• Ac gtgt nI , nQ
• Ac ltlt nI , nQ
• P1 noves to the origin and random phase is
  observed

is around
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Alternatevely

• Clicks are heard after the low pass filter

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Threshold Effect

• As ? is decreased the rate of clicks grows
• Rate of clicks is high threshold occurs

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Designing an FM System

• Given D (?)
• Compute BT
• Given BT and N0 (Noise power per unit bandwidth)
• Determine AC to keep above the threshold

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FM Threshold Reduction

• FM demodulator with negative feeback (FMFB) or PLL

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FM Threshold Reduction
The VCO output
The phase comparator output
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FM Threshold Reduction (cont)
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FM Threshold Reduction (cont)
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FM Threshold Reduction (cont)
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Linear Model of the PLL-FM Demodulator
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PreEmphasis - Deemphasis

• Pre at transmitter
• De- at the receiver

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Pre-emphasis De-emphasis

• Pre at transmitter
• De- at the receiver

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Conclusions